1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor, and particularly to an image capturing apparatus such as a digital video camera and a digital still camera for carrying out automatic focusing and to a control method therefor.
2. Description of the Related Art
A contrast AF method for carrying out automatic focusing contrast information of a subject image captured by an image capturing device is an automatic focusing method widely used by image capturing apparatuses. In the contrast AF method, only a particular frequency component is extracted by bandpass filter processing from a pixel signal obtained from the image capturing device, and the in-focus state of the image capturing apparatus is detected using the frequency component as a focal evaluation value.
In the automatic focusing based on the contrast AF method, a signal of each of RGB color components obtained by the image capturing device may be subjected to white balance processing (WB processing) to be converted into a brightness signal, and thereafter bandpass filter processing may be performed. This is because, for example, when the output signals of the color components are different in the signal intensity, an amplification factor for an output signal of which signal intensity is low is increased as compared with an amplification factor for an output signal of which signal intensity is high, whereby the output signal of which signal intensity is low is complemented, and evaluation is performed without color deviation.
More specifically, in order to normalize the signal intensity of the output signal of each color component, an output signal of a color component of which signal intensity is low is multiplied by a white balance coefficient having a high value, and an output signal of a color component of which signal intensity is high is multiplied by a white balance coefficient having a low value.
With such a method, when in-focus state of a subject having multiple color components is detected with an optical system having a high on-axis chromatic aberration caused by the performance of the image capturing optical system of the image capturing apparatus, only colors having high signal intensity may be simply evaluated, which can reduce the difference of the in-focus position thus caused.
For example, Japanese Patent Application Laid-Open No. 2007-329687 (no corresponding publications in foreign countries) explains white balance processing on the image explained above in detail.
As explained in Japanese Patent Application Laid-Open No. 2007-329687, the white balance coefficient used for the white balance processing includes a value of a ratio between an average of all signal intensities in the AF evaluation area (AveYi of Japanese Patent Application Laid-Open No. 2007-329687 (no corresponding publications in foreign countries)) and an average of the signal intensity of each color component (AveR, AveG1, AveG2, AveB of Japanese Patent Application Laid-Open No. 2007-329687), and accordingly, this complements the difference of the signal intensities of the output signals of the color components, making the AF evaluation possible by eliminating the deviation of the colors. This processing is referred to as normalization.
However, when the conventional technique disclosed in Japanese Patent Application Laid-Open No. 2007-329687 is applied to a low illuminance, an output signal of a color originally having an extremely low signal intensity is multiplied by an extremely high white balance coefficient, which unnecessarily amplifies a noise component of the original output signal.
More specifically, the ratio of the noise component of an output signal having a small signal intensity is high, and accordingly, when an output signal having a high ratio of the noise component is multiplied by an extremely high white balance coefficient in order to normalize the signal intensity of the output signal of each color component, the noise component of the original output signal is unnecessarily amplified, which causes a problem in that the accuracy of the focal evaluation value is reduced.